In performing navigation based on inertia sensors, e.g. acceleration or angular velocity sensors, (inertia navigation), if the sensor signal is being integrated, it is important that the integration time is not extended too much, thus excessively increasing the error in position or direction caused by measuring errors of the sensor. In order to prevent that, the aim often is to divide the motion into periodically repetitive cycles of sufficient brevity. The method is called step-by-step navigation. In athletics coaching and competitions and in fitness exercise and other outdoor activities, such step-by-step navigation is important, wherein e.g. the speed of locomotion, the distance covered, the direction, the step rate (cadence), and the step time, as well as the step length are being measured. The way of locomotion could be e.g. running, walking, pole walking, competitive walking, cross-country skiing, downhill sports, roller skiing, skating or the like, where a cyclic motion is present.
Inertia navigation can work independently, or it can be used in combination with satellite navigation, in order to improve the accuracy of satellite navigation, particularly in areas of poor coverage of the satellite signal, for diagnostic purposes in satellite positioning error situations, or in order to reduce the power consumption of satellite navigation by means of increasing the intervals between instances of reception of the satellite signal.
In prior art, several solutions exist aiming at measuring the distance covered by using an acceleration sensor. In inertia navigation, for example, an acceleration sensor is most often used for measuring the distance covered. By means of the acceleration sensor, the contact time for the foot, i.e. the time during which the foot touches the ground, can be measured. For instance, the U.S. Pat. No. 4,578,769 discloses such a solution according to prior art. The method described in said Patent Publication provides good results for high running speeds, but it is not robust for slow running, nor for walking, where the event of the foot leaving the ground is hard to detect.
The acceleration sensor can be a simple switch or the like, which simply counts the number of steps and estimates the distance based on the number of steps, and the speed based on the cadence. These devices are called pedometers.
As a solution in a slightly more advanced system according to prior art, the actual motion of the walker can be measured at the foot by means of an acceleration sensor. Such solutions according to prior art are disclosed in e.g. the US Patent Application US 2002/0040601, the U.S. Pat. No. 5,955,667 and in the Canadian Patent Publication CA 2,218,242.
In the aforementioned patent publications, measuring signals from a multitude of acceleration sensors and angular motion sensors are combined, and significantly improved precision is achieved compared to the one for pedometers or contact time measurements. In these solutions according to prior art, the drawbacks, however, are the required number of sensors, a linear acceleration sensor as well as an angular motion sensor, for compensating the error caused by the earth's gravitational force, through the inclination and its variation, as well as the complexity of the algorithm, which manifest themselves in the size of the system, its costs, and power consumption.
In order to simplify the measuring system described above, a solution according to prior art has been disclosed, for using an acceleration sensor in such a way, that knowledge of the period of time the foot stays immobile, as it is on the ground, is being utilized and thus, the aim has been to improve the precision through automatic resetting. A solution according to prior art with such a technique is disclosed in e.g. U.S. Pat. No. 6,356,856. The method described in said Patent Publication suffers, however, from inaccuracy, when the inclination changes during the motion. Computing is complex in this case as well, and it requires power and program storage capacity.
One solution according to prior art, for detecting motion and for measuring the duration of movement is a disclosed method based on an acceleration sensor. Such a prior art solution is disclosed, for example, in the U.S. Pat. No. 6,298,314.
One further solution according to prior art, for a general device for measuring the movement of an athlete is disclosed, for example, in U.S. Pat. No. 7,092,846 and in the International Patent Application Publication WO 00/20874.
In literature, several solutions according to prior art, for step-by-step navigation have been disclosed. In all these known solutions, combining simplicity, and thereby low cost, small size, low power consumption and accuracy, is a problem. The major error sources for the presented solutions are the coupling of gravitation to the measuring signal as the angle changes, unambiguous detection of the contact between foot and ground, and foot slipping at ground contact, whereby the acceleration signal and the speed signal are distorted.